The "evidence" seems pretty slim and purely circumstantial on this rumor. On the other hand, as profit margins go, it seems Canon could make a prime cheaper than a zoom, with fewer pieces of glass, and less going on mechanically (no zooming), so their profit margin could be easier to achieve with the prime.
Also, given the sharpness advantages inherent in the simpler primes vs. the more expensive zooms, they may be able to achieve "decent-ness" (if not heavenly-inspired greatness) in IQ without really having to go all-out with super-expensive means in order to do it.
They may also feel that if they don't, then the others will. Sigma in particular should give them a little concern given their recent successes and good press, which gives them more "cred" than the Tokinas and Bowers, etc.. Sigma now sort of rivals OEM lenses in their public perception of quality and value. (at this point, some will disagree loudly) If the rumors of new Sigma 400, 500 and 600mm lenses are true, or if Canon just thinks they may be true, maybe Canon are feeling the pressure to respond and not simply surrender ground to Sigma by default?